Metal halide high-pressure discharge lamps require starting pulses to initiate the discharge across spaced electrodes. The efficiency of energy conversion to light obtainable from such lamps is high, with good color temperature. These lamps do not, however, provide light immediately upon energization but, rather, require some starting time.
A metal halide high-pressure discharge lamp has been describefd in U.S. Pat. No. 3,519,872, Ward, in which a fine heating wire made of coil-coiled tungsten is serially connected with the main electrode. The respective free ends of the electrode and of the heating coil are connected to a current supply, each, which is, electrically insulated from each other, conducted through a pinch or press seal or melt seal of the discharge vessel. The arrangement improves the ignition characteristics of the lamp since heat which can be obtained from the heating coil heats the discharge electrode. Fill condensates which possibly precipitated on the electrode are thereby vaporized. The fine heating wire, typically a fine tungsten wire, cannot, in high-power lamps, heat the entire discharge vessel. Thus, other than facilitating ignition, due to more rapid stabilization of the discharge, no further advantages can be obtained thereby.
Metal halide high-pressure lamps are used more and more for general purpose illumination and, also, have been proposed as headlamps for vehicular use. The power rating of such lamps is low--100W and less, and, especially, 50W and less. The color temperature of such lamps must be acceptable and pleasing to users; additionally, the light output to be obtained from the lamps should rapidly occur upon energization of the lamps.
The discharge vessel of low-power lamps of this type is very small. A typical volume is in the order of about 0.03 cm.sup.3. It is not possible to introduce heater coils or heater windings in such small discharge vessels. Lamps with high power, and having substantial discharge vessels, may have heater wires placed therein.